METHODS OF VENTILATION. 281 



that all is right. It is certainly desirable to employ mechan- 

 ical contrivances, whenever they can, as in the present case, be 

 applied advantageously. But mechanism can never make a 

 gardener, inasmuch as the chief part of what constitutes a real 

 gardener springs from mental, not physical, activity. It is a 

 very easy matter to open and shut the ventilators of a hot- 

 house ; but it requires something more than mere mechanical 

 power to do so with certain benefit to the inhabitants within. 



This will be rendered clear by a common illustration. Let a 

 dwelling-room be warmed to a temperature of 60 ; and suppose 

 it to be tolerably well filled with individuals, by the animal heat 

 and respiration of whom the room by and by becomes somewhat 

 raised in temperature, and contaminated in its atmosphere. 

 Then, all at once, let the windows be thrown open, and the con- 

 sequence is not only disagreeable, but highly dangerous, as is 

 manifest by the murmur which very soon pervades the assem- 

 bled party. Now, the case is precisely similar in a hot-house, 

 only with this difference, the unfortunate plants cannot speak 

 in audible sounds to tell the injuries that are perpetrated upon 

 them ; yet they bear a language, imprinted on their leaves, no 

 less truthful, nor less understood by the attentive observer. The 

 above common occurrence is a plain illustration of what I have 

 often seen, and have been forced to perform, in the ventilation 

 of forcing-houses, and which is more likely to be exemplified by 

 the compound methods which I have described. Science may 

 enable us to be more watchful of atmospheric phenomena, and 

 may draw our attention to facts which mere practice might pass 

 unnoticed. But this is a practical operation which science has 

 not yet approached, and which, in all her discoveries, she never 

 can approach, i. e., to tell us the precise quantum of air to 

 admit at different times and under different temperatures. The 

 method of mixtures does not come near it, and the combination 

 of gases gives the gardener little scientific assistance. We must 

 know the nature and properties of air at all times and tempera- 

 tures ; but the quantities and proportions in which we are to 

 admit it must be learned by experience and strict observation. 

 We must watch its effects upon the plants, and admit it in 

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